My family went on a Star Trek kick recently and I was trying to make something related so I made this to simulate the Star Trek red alert.
First I looked online to find a circuit diagram but I couldn't find any working projects that didn't use an audio recording module. (which doesn't count!)
So I had to make my own that sounded fairly authentic, (by which I mean that I listened to the red alert in videos while adjusting the circuit) and I think I succeeded!
Step 1: Materials
The things you will need to build this are as follows:
- Power supply 5V - 15V
- 555 timers x 2
- PNP transistor (BJT)
- NPN transistor (BJT)
- Red LED
- Select switch
- 10KΩ x 4
- 1KΩ x 4
- 10KΩ potentiometer x 2
- 100nF x 5
- 100µF x 2
- A bypass capacitor is recommended
A voltmeter is not required but may help with circuit debug.
Step 2: Making It
Put the parts together as in the schematic above.
Then you will need to adjust the potentiometers until you find the right frequency, for me that was at 1/3 of the rotation and 2/3 of the rotation respectively.
Step 3: Theory
Now here comes the theory, if you are too scared (of theory) or already understand how it works then you can be finished!
First you need to understand the 555 timer.
The first 555 timer is an astable multi-vibrator that is controlled by the voltage from the switch, when the voltage is high then it operates normally, but when the voltage is low it inhibits the operation of the 555 using the reset pin.
Then when the output of the 555 goes high it charges the capacitors through the diode, which as they discharge slowly through the 10KΩ resistor (labeled R7) they create the changing tone you hear.
When the first 555 is high then the second is inhibited, but when it goes low the second is enabled to create the tones determined by the capacitor's charge level.
Now use your voltmeter to measure the voltage at the base of the PNP transistor, as the LED flashes you should be able to see the change in the voltage.
Now put your meter's leads in series with the speaker and measure the current through the speaker.